(1) Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus, and especially relates to a technology for reducing an artifact (a virtual image) which is contained in an ultrasound image.
(2) Description of the Related Art
An ultrasonic diagnostic apparatus, which enables observing an object to be tested in a noninvasive and real time manner, has had an irreplaceable existence in the medical field. The ultrasonic diagnostic apparatus is an apparatus which sends an ultrasonic pulse that is generated by a probe (a search unit) to the object, and visualizes a condition in the object into an image according to a reflected ultrasonic echo. (For example, see pp 7-20 and pp 24-25 in “Ultrasonic Medical Science TEXT Basic Ultrasonic Medical Science” written by Itoh and Hirata in April, 1998 published by Ishiyaku Shuppan Co., Ltd.)
The following explains a configuration of a conventional ultrasonic diagnostic apparatus and the actions which are taken by the conventional ultrasonic diagnostic apparatus.
FIG. 1 is a block diagram that shows a functional configuration of the conventional ultrasonic diagnostic apparatus 70.
As shown in FIG. 1, the conventional ultrasonic diagnostic apparatus 70 is composed of a search unit 71, a send/receive switching unit 72, a sending unit 73, a beam forming and adding unit 74, a filtering unit 75, a detecting unit 76 and a display unit 77.
The search unit 71 is an apparatus that sends an ultrasonic pulse to an object to be examined, and receives an ultrasonic signal (hereinafter referred to as an “ultrasonic echo”) that is reflected from the object. When the search unit 71 sends the ultrasonic pulse, the search unit 71 receives a pulse signal (hereinafter referred to as a “transmission pulse signal”) to generate the ultrasonic pulse from the sending unit 73, and generates the ultrasonic pulse based on the transmission pulse signal. On the other hand, when the search unit 71 receives the ultrasonic echo, the search unit 71 converts the ultrasonic echo that is reflected from the object into an electric signal (hereinafter referred to as a “received echo signal”), and outputs the electric signal to the send/receive switching unit 72.
When the ultrasonic pulse is sent from the search unit 71, the send/receive switching unit 72 connects the search unit 71 with the sending unit 73. On the other hand, when the ultrasonic pulse is received, the send/receive switching unit 72 switches the search unit 71 to connect with the sending unit 73. When the ultrasonic pulse is sent, the sending unit 73 generates the transmission pulse signal, and outputs the transmission pulse signal to the send/receive switching unit 72.
The beam forming and adding unit 74 executes focusing and all necessary beam formation and addition to the received echo signal that is received from the search unit 71 via the send/receive switching unit 72, and outputs the received echo signal to the filtering unit 75. The filtering unit 75 executes a filtering process to the received echo signal that is output from the beam forming and adding unit 74. The detecting unit 76 executes envelope detection to the received echo signal, which has been processed through the filtering process and outputted from the filtering unit 75, and outputs the received echo signal after the detection process (hereinafter referred to as a “received detection signal”) to the display unit 77. The display unit 77 generates an ultrasonic image based on the received detection signal that is output from the detecting unit 76.
Actions which are taken in the conventional ultrasonic diagnostic apparatus 70 will now be explained. For sending the ultrasonic pulse to the object, the transmission pulse signal is generated in the sending unit 73. The ultrasonic pulse which is generated based on this transmission pulse signal is sent to the object from the search unit 71. The ultrasonic pulse which is sent to the object is reflected at a sound impedance boundary within the object, and comes back to the search unit 71 with a time delay that is caused according to a reflection depth after the transmission has been started. The search unit 71 converts the received ultrasonic echo into the received electric echo signal, and outputs the received echo signal to the beam forming and adding unit 74. The beam form and adding unit 74 corrects the difference in the receiving time when each of the oscillators composing the search unit 71 receives the ultrasonic echo, and executes focusing on the received echo signal.
In addition, because the received echo signal after the beam formation and addition contains some noise components, a filtering process by a band pass filter (BPF) is executed for efficiently extracting the received echo signal in the harmonics frequency, which is a central frequency or twice of the central frequency in the filtering unit 75. The received echo signal that has been through the filtering unit 75 is multiplexed by using a Hilbert conversion filter in the detecting unit 76. Then, after the received echo signal is multiplexed, by applying the envelope detection to the received echo signal, the echo signal is converted into the received detection signal which shows luminance for generating an ultrasonic image, and the like. Lastly, the display unit 77 receives the detection signal from the detecting unit 76, generates the ultrasonic image based on the received detection signal, and displays the generated ultrasonic image on a display apparatus, or the like.
For the ultrasonic diagnostic apparatus, an ultrasonic pulse having a desired directivity (for example, a feature to have a stronger acoustic pressure in a front direction) is used for improving the picture quality of the ultrasonic image.
However, the ultrasonic pulse which is actually sent contains a plural number of side lobes that are sent in an undesired direction (for example, 45 degrees in both right and left directions) in addition to the main lobe that is sent in a desired direction (in the front direction).
FIG. 2 is a diagram that shows an overview of a plural number of side lobes 82 and 83, which are generated at both sides of a main lobe 81 and sent in undesired directions (i.e. In right and left oblique directions; hereinafter, such undesired directions are referred to as a “side lobe direction”). Because of these side lobes, the side lobe of the ultrasonic pulse is sent to an object which is located in the side lobe direction. When the ultrasonic diagnostic apparatus 70 receives the ultrasonic echo, the ultrasonic diagnostic apparatus 70 receives the ultrasonic echo that is reflected from the object which is located in the side lobe direction at the same time the ultrasonic diagnostic apparatus 70 also receives the main lobe of the ultrasonic pulse that is reflected from the object which can reflect the ultrasound to be displayed as the ultrasound image. As a result, the ultrasonic echo that is reflected from the object which is located in the side lobe direction is generated as an artifact (a virtual image), and causes a problem to induce misdiagnosis through the ultrasound image containing the artifact (Please see the aforementioned reference book).
FIG. 3 is a pattern diagram showing a process in which the artifact (a virtual image) that is generated by the above-described side lobes 82 and 83 are displayed as the ultrasound image. As shown in FIG. 3, there are objects 91 and 92 that can reflect an ultrasound to be displayed as the ultrasound image, the main lobe of the ultrasonic pulse is reflected from these objects, and the received echo signals 96 and 97 are generated. However, this ultrasonic pulse contains the side lobe. When there are objects 93-95 which are located in both the right and left side lobe directions, which objects are not desired to be displayed as the ultrasound image and from which the ultrasound may be reflected, these objects reflect the side lobe so that the received echo signals 93a-95a are generated. As a result, the signals 93a-95a through the above-described side lobe remain even in the received detection signal, and these appear as the artifact.